Ac generator system including differential protection of exciter

ABSTRACT

A differential protection system is connected to detect and indicate faults in an exciter by comparing signals developed from the exciter field and from the generator armature. A first signal is developed representing the magnitude of the voltage on the exciter field winding and a second signal is developed representing the vectorial sum of the current and voltage on the armature winding. These signals are compared and a fault indication produced when the magnitude of their difference is at least of a certain value. The differential protection in brushless exciters permits avoiding the use of a fuse in series with each diode of a rotating rectifier.

United States Patent South et al. p Y 1 Dec. 5, 1972 [541 AC GENERATORSYSTEM INCLUDING 3,027,509 3/1962 Lamaster ..322/2s DIFFERENTIALPROTECTION OF 3,371,235 2/1968 Hover...., ...3l/68 R 'EXCITER 3,652,9203/1972 South etal. ..322/19 72 Inventors: William 1!. South McKees ort;I

[ 1 u L. mum"; North Znmary EJxm+neSrJames D.l Trammell v Sailles, ofPattorney- Nation eta I73] Assignee: Westinghouse Electric Corporation,[57] ABSTRACT Pi'ttsburgh,lPa., A differential protect1on system isconnected to detect Filed! Mamh 9.72 and indicate faults in an excit'erby comparing signals [2]] Appl 235376 developed from the exciter fieldand from the generator armature. A first signal is developedrepresenting g Y the magnitude of the voltage on the exciter field wind-[52] US, Cl. ..3l7/l3 R, 317/27 R, 322/25 ing and a Second Signal isdeveloped representing the [51] [I ll- CLf ....-...H02h 7/06 vectorialsum of the current and voltage on e arma [58] held 0 Search 33 152 turewinding. These signals are compared and a fault I 2 indication producedwhen the magnitude of their dif- 6 R f d ference is at least of 'acertain value. The differential 1 e erences protection in brushlessexciters permits avoiding the UNITED STATES PATENTS use of a fuse inseries with each diode of a rotating rectifier. v 1 3,009,090 l 1/1961Stearley et al. ..322/ X 6 Claims, 1 Drawing Figure I o 332 39 32 3 Is22 f 28 $23 A \gzggtA Ersz I iERbfiT-fJSg I E QQQQ '4 222??? GENERATOR1o FIELD ASSEMBLY FIELD EXCITER I 1 ARMATURE 30 32 ;2a so 322 8 3o p 28*2; F: *3; :5 5/32 WA T U AE FgIE W a? 20 J T :94! :5 :a-srz L3? 22 34 3ALARM OR R fibliifiR 54 i lifiii PATENTED DEC 5 I972 I 1 H mm, I mm/- m9 0m mm L v M?) wV JV? ON 97" mm\ [J N fi I 5:8 mwtuxm B Z BACKGROUND OFTHE INVENTION l. Field of the Invention i This invention relates toelectrical generation systems. g I

" 2. Prior Art In present generator excitation systems, if a faultoccurs in the exciter, there is no remote indication of any.-

trouble. Serious faults can occur from a variety of causes in thevarious known types of exciters aud endanger the entirepsystem if it isnot promptly shut down.

A form of exciter is that known as a brushless exciter in which the ACarmature winding of theexcite'r and-a rectifier assembly rotate on thesame shaft for "direct supply of field current to the generator fieldwinding without slip rings .or brushes.- Normally several semiconductor"diodes are connected in parallel for each A.C. phaseand as usedheretofore each diodehas a separate fuse element in series therewith soas to avoid a direct short between the-exciter armature and thegenerator field upon a shorting failure of thediode, towhich'semiconductor diodes are susceptible. l-loover US. Pat. No.3,37l,235 is illustrative as to known brushlessexciters. It can be seenthat the use of a fuse in series with each diode can be expensive bothin terms of the components themselves and because of the additionalrotating mass. Such fusescompletely isolate an individual shorted diodeand do not provide protection for any fault occurring ahead of thefuses, e.g., in the exciter armature winding. If the latter type offault is present, although. in present systems there is' a very lowprobability of that, destruction of the exciter can occur or the'voltageregulator tries to work'harde'r but may not, as such voltage regulatorsare presently devised, give a forcing alarm. v I

Consequently, the prior art does not provide exciter systems thatareprotected against catastrophic failure that could otherwise occur inrare instances.

SUMMARYQF THE INVENTION In-accordance with this invention, adifferential protection system is connected to detect and indicatefaults in an exciter by comparing signalsdeveloped from theexciter-fieldand from the generator armature. A first signal isdeveloped representing the magnitude of the voltage on the exciter fieldwinding and a second signal is developed representing the vectorial sumof the current and voltage on the armature winding. These signals arecompared and a fault indication produced when the magnitude of theirdifference is at least of a certain value.

The effect of gain in the exciter is compensated for by a resistiveimpedance connected within the means for developing the referred tofirst signal. Also, the effect of the synchronous reactance of thegenerator is compensated for by a resistive impedance connected acrossthe means developing the second referred to signal. These resistiveimpedances can be made adjustable to'provide a single differentialprotection unit that can be adapted for various machine characteristics.i

It is desirable to include, within the means for comparing the developedsignals, a time delay means to in sure the required minimum differencebetween the signals endures a given minimumtime before a fault isindicated. v

Although the differential protection scheme of the invention-may be usedin connection with "any type of exciter, it is especially suitable foruse in'brushless excitation systems where the rotating diodes may now beused without fuse elements inseries with each one but with reliance uponthe differential protection system to provide a fault signal.

THE DRAWING The 'singleFlGURE of the drawing is a cireuit'schej matic ofone embodiment of the present invention.

DESCRIPTION OF THE 'PREFERRE EMBODIMENTS- Referring to the drawing,there is shown an A.C.

generator system including an altematingcurrent generator with aDC'field winding, 10-and an AC armature winding 12 (.here shown as athree phase winding) to which the field winding is inductively related.An exciter 1-3 is connected with the field winding 10 of the generatorto produce DC excitation thereon. While various types of exciters may beused, the description of the invention will be presented in connectionwith a brushless system in which the unit comprising the exciterarmature 14, a rotating rectifier assemb1yl6, and the generator fieldwinding 10 are parts of a commonly rotated unit 18. The exciter 13 alsohas a DC field winding 20 that is part of the stationary structure. I

In the example shown, the exciter armature 14 receives its fieldenergization from a pilot exciter that includes a permanent magnet fieldmember 22 in the rotating-unit 18 that is electromagnetically coupled toa stationary armature 24 to apply apilot voltage to voltage regulator26. The regulator 26 may be of a known type to apply a controlled DCvoltage to the exciter field winding 20. v

least one diode 28 for rectification of each'half cycle of the AC wavein each phase. In accordance with known practice, each of the diodes 28may have connected across it a voltage suppressor capacitor 30 and aresistor 32. Y v

The apparatus also includes a differential protection system 34 inaccordance with this invention. This system includes a first means 36 todevelop a first signal representing the magnitude of the voltage on theexciter field windingrThis first signal may be developed either bydirect connection of lines 37 and 39 across the exciter field winding,as shown, or by development of a current across a low resistance inseries with the exciter field winding. Resistance 41, which may beadjustable, is connected across lines 37 and 39 to compensate forexciter gain.

There is also provided a second means to develop a second signalrepresenting the vectorial sum of the current and voltageof one phase ofthe AC armature windtransformers are joined at point 44 to produce'avectorially added signal in opposition to the signal developed by means36. The current transformer 42 is connected across a resistance 46,which may be adjustable, to compensate for the synchronous reactance ofthe generator.

An A.C. signal developed by the transformers 40 and 42 on lines 48 and49 is rectified, such as by the rectifi er bridge 50 which is preferablyfollowed by a filter capacitor 52, before combination with the DC signaldeveloped from the field of the exciter.

The signals that are developed for the differential protection systemare signals normally developed for use in the voltage regulator 26 andthe same sensing units may be used for both the differential protectionsystem 34 and the voltage regulator 26.

The result of the combination thus far described is to provide on lines54 and 55, extending respectively from the tap on the resistor 41 in thefirst sensing means and one side of the filter capacitor 52, a signalthat represents the difference in the two generated signals from means36 and 38.

A differential relay 56 is provided that is responsive to the differencesignal when the latter is of sufficient magnitude. In this embodimentthe differential relay 56 has a second rectifier bridge 58in front of itacross lines 54 and 55. The rectifier bridge 58 ensures operability ofthe system regardless of the polarity of the sensed difference signal.Coil 59 of the relay 56 has a transistor 60 in series with it to a point61 of reference potential. A Zener diode 62 is connected to the base oftransistor 60. A resistance 64 and capacitance 66 are connected betweenthe input side of coil 59 and point 61 for introducing a predeterminedtime delay in order to insure that the unit 34 is not susceptible tobrief transients that are of such limited duration as to be tolerable.The cathode of Zener diode 62 is connected to a point between elements64 and 66. Consequently, the voltage is permitted to build upon thecathode of the Zener diode 62 for thetime constant of the time delayelements 64 and 66. When the Zener diode 62 breaks down it is only thenthat the transistor 60 turns on to provide a conductive path through therelay coil 59. Contacts 68 of relay 56 are then closed to cause a faultindication signal to be supplied to alarm or shutdown mechanism 70 toshut down the exciter or otherwise provide indication to personnel thata fault exists. Other known configurations of elements may be used toenergize the relay 56.

The differential protection system 34 provides a means to monitorcontinually the condition of the exciter armature l4 and rotatingrectifier assembly 16 in brushless exciters. Although system 34 may beused with other types of exciters, it is particularly advantageous inthe illustrated application because it permits doing without a fuse inseries with each diode 28. Thus in preferred forms of the invention thediodes 28 are directly connected, without fuses, between the exciterarmature winding 14 and the generator field winding 10. The cost offuses is high enough, and the reliability of present diodes is highenough, that the risk of having the machine shut down by a diode failuremay be worth running. In past brushless exciters, the fuse would isolatea single faulted diode in a parallel con- 4 nec tion and th e machinecould keep operating alt ough periodic maintenance was necessary toreplace fuses.

The circuit as described does not take into account the effect ofsaturation on either the exciter or generator. Normally this effectshould not require consideration because the saturation effects can beapproximated by a straight line (i.e., linear circuit elements).However, it may conceivably be desirable, on some units, to obtaingreater sensitivity by adding non-linear circuit elements to correct forthe non-linear saturation of the machine. These non-linear elementscould be added in series with the output of either element 41 and 52, orboth. A Zener diode non-linear element could be used similar to themanner in which a Zener diode is used in South US. Pat. No. 3,590,277.

We claim:

1. An A.C. generator system including differential protection of exciterelements comprising: an altemating current generator having inductivelyrelated D.C. field winding and A.C. armature winding; an exciterconnected with said field winding of said generator, said exciter havinga DC. field winding; a differential protection system including firstmeans to develop a first signal representing the magnitude of thevoltage on said exciter field winding, second means to develop a secondsignal representing the vectorial sum of the current and voltage of saidA.C. armature winding, and third means to produce a third signalrepresenting the difference in magnitude of said first and secondsignals.

2. The subject matter of claim 1 wherein: said first means includes afirst resistive impedance of a magnitude to compensate for gain of saidexciter; said second means includes a second resistive impedance of amagnitude to compensate for the synchronous reactance of said generator;and said first and second signals are both direct currents.

3. The subject matter of claim 2 wherein: said third means includesmeans responsive to a given minimum level of difference between saidfirst and second signals to produce a fault indication, and said thirdmeans in cludes time delay means to limit the response of saidresponsive means to instances in which said minimum level of differencebetween said first and second signals endures for a given minimum time.

4. An A.C. generator system comprising:

an alternating current generator having inductively related D.C. fieldwinding and A.C. armature winding; a brushless excitation systemincluding an alternating current exciter and a rotating rectifierassembly, said rotating rectifier being connected with said generatorfield winding, said exciter having a DC. field winding and an A.C.armature winding; a differential protection system connected to detectand indicate faults in said exciter by comparing signals developed fromsaid exciter field and from said generator armature.

5. The subject matter of claim 4 wherein: said rotating rectifierassembly comprises at least one semiconductor diode directly connectedbetween said exciter armature winding and said generator field winding.

6. The subject matter of claim 5 wherein: the direct connection of saiddiode to said windings is without any fuse element therein.

1. An A.C. generator system including differential protection of exciterelements comprising: an alternating current generator having inductivelyrelated D.C. field winding and A.C. armature winding; an exciterconnected with said field winding of said generator, said exciter havinga D.C. field winding; a differential protection system including firstmeans to develop a first signal representing the magnitude of thevoltage on said exciter field winding, second means to develop a secondsignal representing the vectorial sum of the current and voltage of saidA.C. armature winding, and third means to produce a third signalrepresenting the difference in magnitude of said first and secondsignals.
 2. The subject matter of claim 1 wherein: said first meansincludes a first resistive impedance of a magnitude to compensate forgain of said exciter; said second means includes a second resistiveimpedance of a magnitude to compensate for the synchronous reactance ofsaid generator; and said first and second signals are both directcurrents.
 3. The subject matter of claim 2 wherein: said third meansincludes means responsive to a given minimum level of difference betweensaid first and second signals to produce a fault indication, and saidthird means includes time delay means to limit the response of saidresponsive means to instances in which said minimum level of differencebetween said first and second signals endures for a given minimum time.4. An A.C. generator system comprising: an alternating current generatorhaving inductively related D.C. field winding and A.C. armature winding;a brushless excitation system including an alternating current exciterand a rotating rectifier assembly, said rotating rectifier beingconnected with said generator field winding, said exciter having a D.C.field winding and an A.C. armature winding; a differential protectionsystem connected to detect and indicate faults in said exciter bycomparing signals developed from said exciter field and from saidgenerator armature.
 5. The subject matter of claim 4 wherein: saidrotating rectifier assembly comprises at least one semiconductor diodedirectly connected between said exciter armature winding and saidgenerator field winding.
 6. The subject matter of claim 5 wherein: thedirect connection of said diode to said windings is without any fuseelement therein.